Electroless plating of gold



United States Patent Ofitice 3,300,328 Patented Jan. 24, 11967 3,300,328 ELECTROLESS PLATING OF GOLD Betty M. Luce, Willowick, Ohio, assignor to Clevite Corporation, a corporation of Ohio No Drawing. Filed Nov. 12, 1963, Ser. No. 323,127 15 Claims. (Cl. 117-47) This invention relates to coating of base materials with a relatively thin layer of gold by means of electroless deposition, sometimes referred to as chemical plating.

As is well known, electroless plating in its broadest aspects involves the immersion of abase material in a plating bath and the consequent deposition of a layer of film of the plating metal on the base by means of chemical reaction and without the use of externally applied electrical energy. The chemical reaction taking place is an autocatalytic reduction of metal ions in the aqueous plating bath. In some cases, the base material is flashed or mirrored to coat it with an extremely thin layer of some metal which acts as a sensitizer to initiate the reduction reaction and/ or promote adherence of the electroless deosit.

p The electroless technique has several advantages over electroplating. One of the most important of these is the fact that it produces a uniform plating thickness on even the most intricate shapes whereas electroplating does not throw well into niches and crevices. Among the other advantages of electroless plating are the simplicity of the required apparatus engendered by the elimination of electrical equipment, and the relative ease of plating non-conductive base material such as plastic films.

While there has been considerable activity in recent years directed toward the application of the electroless plating principle to a wide variety of metals, the only commercially satisfactory processes evolved to date, insofar as is known, are for the plating of nickel, cobalt, and copper. Practical success with other metals heretofore has been nil or highly limited. Typical disadvantages characterizing the processes heretofore devised are extremely slow deposition rate, coatings of unsatisfactory integrity and/or adherence, and instability of the plating baths resulting in lack of controllability of the'deposition and rapid depletion of the bath.

It has now been found that metallic gold may be plated onto a metallic or a non-metallic substrate by a method of electroless plating which enables the production of dense, smooth, adherent gold coatings at higher deposition rates than heretofore possible. Moreover, compositions have been developed for the electroless plating of gold from aqueous solution which are relatively quite stable and permit close control and reducibility of the platings produced.

To the accomplishment of the foregoing related ends, said invention, then, consists of the means hereinafter fully described and particularly pointed out in the appended claims, the following description setting forth in detail certain illustrative embodiments of the invention,

such disclosed means constituting, however, but a few of gold at the interface between thesubstrate and the aquegrams per liter).

ous solution is effected by a reducing agent which is selected from the group consisting of hydrazine, alkyl hydrazines in which the alkyl group contains from one to three carbon atoms, hydroxylamine and alkyl hydroxylamines in which the alkyl group contains from 1 to 3 carbon atoms. These aqueous solutions are quite unstable in the absence of a stabilizing agent, and it has been found that the cycloaliphatic amines are a particularly satisfactory class of materials for this purpose.

The term alkali metal as used herein means sodium, potassium, and lithium.

The quality of the plate of the present invention is enhanced during use by agitation of the bath, and particularly utilization of mechanical stirring means which prevents local over-heating of the aqueous electroless gold plating bath.

The plating solutions of the present invention may be contained in any suitable vessel preferably non-metallic, e.g., glass, and the base material or article to be plated suspended by any suitable means such as a clamp, mounting or in a conventional plating basket. Alternatively, the plating solution may be sprayed, brushed or otherwise brought into contact with the surfaces to be plated. From the standpoint of control and maximum utilization of the plating solution, immersion is preferred.

Ultrasonic vibration of the object undergoing plating improves the uniformity and coherence of the gold plate. Any suitable means for supplying ultrasonic energy may be employed, for example when a clamp or holder is utilized to support the object being plated in the aqueous electroless plating solution, such holder may be suitably coupled to an ultrasonic transducer which is in turn connected to a source of high frequency A.-C. electrical potential. A frequency of about 20 kc. is eifective for the purpose of this invention.

It will be understood that a wide variety of specific forms of apparatus may be devised and employed in the performance of electroless plating as contemplated by the present invention.

The chemical constituency of the plating bath varies, of course, with the metal or combination of metals to be deposited. Stated generally, it comprises an alkaline aqueous solution of at least one complexing agent; at least one plating metal ion source of material, soluble in the solution and providing gold ions; and a reducing agent. In general the pH is in the range of from 7 to 12, and most usually in the range of from 8 to 11.

The solution is rendered alkaline by the use of a suitable base, preferably ammonium hydroxide. The operative range of concentration is about 15 to 40 grams per liter or, in any event, high enough to give a pH above 7 and preferably above 10. As the pH drops towards 7, the deposition rate falls off sharply. Satisfactory deposition rates are achieved throughout the operative range, but inasmuch as stability of the solution is inversely related to alkalinity, the pH in the higher portion of the range is preferred, eg a pH of above 10.

The complex-ing agent is a water soluble sulphi-te or meta-bisulphite of ammonium, or an alkali metal, e. g., sodium, potassium, or lithium. The permissible range of concentration of the complexing agent in the aqueous solution is from about 0.7 to 2.5 mols/liter. While variation in the concentration does not substantially affect the rate of deposition, the stability of the solution decreases with decreasing concentration. An optimum condition of stability at deposition rate is achieved in the range of from about 1.2 to 1.6 mols/liter (-200 Higher concentrations tend to cause the salts to crystallize out of solution. 1

The source of the plating metal ions of gold in the bath may be any compatible gold compounds which are soluble in the bath and provide gold ions in solution. For gold plating, the chloride, nitrate, acetate, sulphate, and various acidic and basic salts of gold, e.g., ammonium, chloraurate, aurichloric. acid, auribro mic acid, etc. A preferred water soluble compound productive of gold ions in the electroless plating baths of the present invention is aurichloric acid, HAuCl .3H O. In general, the permissible range of concentration of the gold ions in the aqueous solution is from about 0.013 to 0.05 mol per liter. At a higher concentration of gold, the gold complex formed in situ tends to become insoluble. At a concentration below about 0.013 mol per liter of gold, less satisfactory coverage of the substrate is found. At about 0.025 mol per liter, the maximum rate of 0.0001 inch per 30 minutes of film deposition was obtained. Experience has shown that, as a general rule, an excess of complexing agent is not detrimental to the process. The main requirement is that there is at least one donor group available to coordinate with each metal ion present. For most purposes, a nrol ratio of complexing agent to metal ion is conveniently in the range of from about 5:1 to about 25:1.

The reducing agent employed in the electroless plating solutions of the present invention is a very powerful reducing agent in comparison with the formaldehyde and polymeric formaldehyde reducing agents heretofor used in electroless plating compositions. It has been found that hydrazine, N H and hydroxylarnine, HONI-l and the alkyl derivatives of these materials in which the alkyl substituent contains from 1 to 3 carbon atoms, are particularly well adapted for use as reducing agents in the aqueous electroless plating baths of the present invention. Thus, methyl hydrazine, dimethyl hydrazine, ethyl hydrazine, diethyl hydrazine, propyl hydrazine, dipropyl hydrazine, acet'aldoxime, acetroxamine, propionaldoxime, and the like, are particularly useful in accordance herewith. The amount of the reducing agent which can be tolerated in the solution without introducing undesirable instability factors will be dependent upon the concentration of the other reactant materials in the bath. Generally, the reducing agent is present in an amount ranging from about 0.3 mol per liter to about 2.4 mols per liter. In the case of hydrazine, the material is operative over this range, but the optimum concentration of hydrazine is in the range of from about 1.2 to about 1.8 mols per liter of solution. About a concentration of 1.2 mols per liter, the plating rate utilizing hydrazine as the reducing agent was not effected, but the bath tends to become more unstable.

An increase in solution stability can be secured by incorporating in the compositions of the present invention from about 0.65 to about 1.95 mols per liter of a cycloaliphatic amine such as cyclohe-xylamine, methylcyclohexylamine, cyclopropylamine, cyclobutylaimine, cyclopentylamine, etc. While the cycloaliphatic aminesdo increase the solution stability, they tend to decrease the plating rate as the concentration of the stabilizing agent is increased. For example, in order to stabilize the solution at 85 C. for thirty minutes, 1.3 mols per liter of cyclohexylaimine is required. Additional cyclohexylamine dropped the plating rate from 0.000092 inch per 30 minutes to 0.000052 inch per 30 minutes. Solution stability per periods of from 25 minutes to 75 minutes or more, is regarded as satisfactory.

As indicated above, the pH range of the solutions of the present invention should be on the alkaline side, and broadly range from a pH of 7 to a pH of about 12. Ammonium hydroxide is a convenient alkalizing agent from adjusting the pH to the desired range although sodium hydroxide and potassium hydroxide may be used. An increase in the pH increases the plating rate considerably, but the bath tends to become more unstable as the temperature is increased. Thus, the higher the pH,

the lower the temperature which is to be used for carrying out the electroless plating opera-tion. Although an increase in temperature f avors bath decomposition, elevated temperatures are preferred for the autocatalytic reaction which occurs at the interface between the surface on which the deposition is taking place and the aqueous electroless plating bath.

As indicated above, agitation increases the rate of plating without substantially affecting the stability of the bath.

The substrate may, as above indicated, he metallic or non-metallic, such as a synthetic resinous film. In any event, best results are secured when the substrate whether metallic or non-metallic is previously sensitized by the deposition thereon of a very thin layer of a catalytic metal or discrete activating sites or nuclei, which may be, for example, nickel, cobalt, copper, silver, rhodium, gold, platinum, or palladium. These metal surfaces or catalytic nuclei may be deposited by any convenient means such as, electroplating, vaporization, or electroless plating, and provide a condition at the interface between the plating bath and the surface being plated which is receptive to and promotes the autocatalytic decomposition of the gold ions to metallic gold. The most preferred of these sensitizing materials is nickel, which has proved to be a very effective catalyst, even more so than gold.

It becomes convenient at this point to illustrate further the present invention by setting forth the preferred mode of carrying out the invention together with alternative modes, it being understood that these are merely illustrative of the invention and are not to be construed as limiting the invention to the precise scope thereof. Those skilled in the art following the direction and suggestions contained in the following examples will be able to proguce still further examples of gold electroless plating aths.

Examplel Mols per liter HAuCl -3H O 0.025 Sodium sulphite 1.6 Ammonium chloride 1.5 Cyclohexylamine 1.3 Hydrazine 1.8 Water, to make one liter.

The substrate used in this particular example was an electroless nickel plated steel sheet. The deposition of gold from the electroless gold plating bath was carried out at a temperature of C. with agitation and yielded a deposition rate of 0.00015 inch per hour. The solution of Example I is quite satisfactory for depositing 0.0001 inch to 0.002 inch of gold on a surface without continuous filtration. The solution life may be prolonged by employing continuous filtration of the plating bath during the course of the deposition. The foregoing bath of Example I represents the best compromise between rate of deposition of gold from the solution and bath stability under the conditions of plating. Solutions pursuant to the formulation of this example have stabilities in excess of one hour up to six hours.

Example II Mols per liter HAuCl -3H O 0.013 Sodium meta-bisulphite 1.0 Ammonium chloride 1.5 Cyclohexylamine 1.3 Hydrazine 1.8

Water, to make one liter.

The above solution is satisfactory for depositing 0.0001 inch to 0.0002 inch of gold on a nickel sensitized steel surface of 85 C. with mechanical agitation. Continuous filtration of the bath through a sintered glass filter improves the stability of the solution.

Example III 7. An electroless gold plating bath in accordance with Mols per liter claim 1 in which the reducing agent is hydrazine.

HAuBr -3H O 0.05 8. An electroless goldplating bath in accordance with Ammonium sulphite 1.3 claim 1 in which the reducing agent is hydr-oxylamine. Cyclohexylamine 1.5 5 9. An electroless gold plating bath in accordance with Hydroxylamine 2.0 claim 1 in which the stabilizing agent is cyclohexylamine. W t to make one liter. 10. An electroless gold plating bath in accordance with The above solution is satisfactory for deposition 0.0001 Claim 1 in which the Complexihg agent is Sodium sulphite inch to 0.0002 inch of gold on a silver sensitized Mylar the reducing agent is hydrazine, and the stabilizing agent plastic film at 85 C. with vigorous agitati-on by bubbling is cyclohfiXylaminei i h b h 11. An electroless gold plating bath in accordance with The following table illustrates further electroless gold claim 1 in which the complexing agent is sodium metaplating aqueous solution and examples of deposition rates bisulphite, the reducing agent is hydrazine and the stabilobtained. izing agent is cyclohexylamine.

TABLE Example Moles/liter, Gold Ion Source Decomposition N 0. Moles/liter, Agent Moles/liter, Reducing Agent Moles/liter, Stabilizer Rate, .001 in./

min. 85 C.

ifo-ms oinlu. re-uniso 1.8-Hydrazine 1.3-Cycl0hexylamine 0. 022 do .d0 0.026 (10 0.048

0.3-Hydrazine 0. 024 0.6-Hydrazine- 0. 034

1 .Z-Hydrazine 0. 047 QA-Hydrazine. 0. 048 1.8-Hydrazine. 0. 009 .-do 0.100

. 0. 112 Cyclohexylamine. 0. 110 .3-Cycl0hexylamine. 0. 092 .95-Cyclohexylaminc 0. 052

Lfi-dimethylhydrazine LS-phenylhydrazine Other ingredients which do not adversely aifect the plating of gold from the solution or the character of the film and its adhesion to the substrate may be included in the composition. Solution stability is also improved by continuous filtration of the plating bath during plating to remove suspended catalytic nuclei from the solution. Preferred practice involves cooling the bath as it leaves the plating tank, filtering, and reheating prior to circulation back to the plating tank.

Other modes of applying the principle of this invention may be employed instead of those specifically set forth above, changes being made as regards the details herein disclosed, provided the elements set forth in any of the following claims, or the equivalent of such be employed.

It is, therefore, particuluarly pointed out and distinctly claimed as the invention:

1. An electroless gold plating bath consisting essentially of an aqueous solution of a complexing agent selected from the group consisting of alkali metal sulphites, ammonium sulphite, alkali metal meta-bisulphites and ammonium meta-bisulphite, a water soluble source of gold ions, a reducing agent selected from the group consisting of hydrazine, alkyl hydrazines in which the alkyl group contains from 1 to 3 carbon atoms, hydroxylamine, and alkyl hydroxylamines in which the alkyl group contains from 1 to 3 carbon atoms, and a cycloaliphatic amine stabilizing agent.

2. An electroless gold plating bath in accordance with claim 1 having a pH of at least 8.

3. An electroless gold plating bath in accordance with claim 1 in which the complexing agent is an alkali metal sulphite.

4. An electroless gold plating bath in accordance with claim 3 in which the complexing agent is sodium sulphite.

5. An electroless gold plating bath in accordance with claim 1 in which the complexing agent is an alkali metal meta-bisulphite.

6. An electroless gold plating bath in accordance with claim 5 in which the complexing agent is sodium metabisulphite.

12. An electroless gold plating bath consisting essen tially of the following constituents in aqueous solution on a per liter basis:

Mols per liter Sodium sulphite 0.7-2.5 Hydrazine 0.3-2.4 Gold 0.013-005 Cyclohexylamine 0.65-1.95

13. An electroless gold plating bath consisting essentially of the following constituents in aqueous solution on a per liter basis:

Mols per liter HAuCl -3H O -2 0.025 Sodium sulphite 1. Ammonium chloride 1.5 Cyclohexylamine 1.3 Hydrazine 1.8

References Cited by the Examiner UNITED STATES PATENTS 2,976,181 3/ 1961 Brookshire. 3,032,436 5/ 1962 Gostin et al. 3,123,484 3/1964 Pokras et a1. 3,214,929 10/ 1965 Edson.

ALFRED L. LEAVITT, Primary Examiner.

R. S. KENDALL, Assistant Examiner. 

1. AN ELECTROLESS GOLD PLATING BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF A COMPLEXING AGENT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL SULPHITES, AMMONIUM SULPHITE, ALKALI METAL META-BISULPHITES AND AMMONIUM META-BISULPHITE, A WATER SOLUBLE SOURCE OF GOLD IONS, A REDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF HYDRAZINE, ALKYL HYDRAZINES IN WHICH THE ALKYL GROUP CONTAINS FROM 1 TO 3 CARBON ATOMS, HYDROXYLAMINE, AND ALKYL HYDROXYLAMINES IN WHICH THE ALKYL GROUP CONTAINS FROM 1 TO 3 CARBON ATOMS, AND A CYCLOALIPHATIC AMINE STABILIZING AGENT.
 15. THE PROCESS OF AUTOCATALYTICALLY PLATING GOLD ON A NON-METALLIC SUBSTRATE WHICH COMPRISES SENSITIZING SAID SUBSTRATE BY DEPOSITING A FILM OF A METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL, COBALT, COPPER, SILVER, RHODIUM, GOLD, PLATINUM, AND PALLADIUM ON SAID NONMETALLIC SUBSTRATE, AND THEREAFTER CONTACTING THE SENSITIZED NON-METAL SUBSTRATE WITH A SOLUTION ACCORDING TO CLAIM
 1. 